1. Field of the Invention
This disclosure relates to a sheet feeding unit configured to feed sheets, and an image forming apparatus provided with the sheet feeding unit.
2. Description of the Related Art
Examples of image forming apparatus such as an electrophotographic copying machine include an apparatus configured to allow a sheet feeding cassette containing sheets to be supplied to an image forming unit in a stacked manner to be inserted in and drawn out from a main body of the image forming apparatus (hereinafter, referred to as an apparatus body). In the image forming apparatus, a feeding roller moves downward to a feeding position for feeding the sheet in association with mounting of the sheet feeding cassette and comes into abutment with the sheet in the sheet feeding cassette. Then, in accordance with a feeding signal, the feeding roller rotates in a sheet feeding direction, sheets are fed to downstream from a topmost sheet in sequence.
Some of the image forming apparatus are configured in such a manner that the feeding roller moves upward to a waiting position with the sheet feeding cassette in a state of being drawn out, and the feeding roller moves downward in mid-course of completion of mounting of the sheet feeding cassette on the apparatus body. In this configuration, however, the feeding roller may be moved downward to the feed position before the completion of mounting of the sheet feeding cassette to the apparatus body, thereby coming into press-contact with a topmost sheet in the sheet feeding cassette.
In such a case, the sheet feeding cassette is pushed further inward with the feeding roller in a state of being in press-contact with the topmost sheet. Accordingly, wrinkles may be formed on the topmost sheet and the posture of the sheet gets out of alignment. In particular, in the case where the direction of movement of the sheet feeding cassette matches an axial direction of the feeding roller, a phenomenon of formation of wrinkles notably occurs because a sheet is pinched between a side regulating plate configured to regulate side ends of the sheets and the feeding roller. If the sheet is fed in a state of being out of alignment, the wrinkles of the sheet are collapsed by the roller and hence become more apparent, or the sheet are skewed. Consequently, feed failure may result.
In the related art, Japanese Patent Laid-Open No. 2004-292093 discloses an elevating mechanism of a pickup roller (feeding roller) including a sheet feeding cassette. The sheet feeding cassette includes a first lever configured to be moved in conjunction with a mounting action with respect to a main body of a sheet feed unit, a second lever configured to support the pickup roller via the first lever, and a torque limiter including a coil spring disposed on a coupling portion with respect to the first and second levers. In this sheet feed unit, if the sheet feeding cassette is inserted into the main body of the sheet feed unit, the inserted sheet feeding cassette and the first lever come into abutment with each other, and the abutment pivots the first lever in association with the movement of the sheet feeding cassette in a mounting direction. At this time, the coil spring that couples the first lever and the second lever is increased in diameter if the first lever rotates by being pressed by the sheet feeding cassette. Therefore, the pivotal movement of the first lever is not transmitted to the second lever. If the first lever is further pivoted and the support of the second lever is released, the second lever pivots downward under its own weight and the inner diameter of the coil spring is reduced. Consequently, the first and the second levers move in conjunction with each other to move the pickup roller downward.
In this manner, the sheet feeding unit of Japanese Patent Laid-Open No. 2004-292093 includes the torque limiter to retard timing of pivot of the second lever with respect to timing of abutment between the sheet feeding cassette and the first lever and thus retard timing of downward movement of the pickup roller. However, since the torque limiter used here includes the coil spring, the timing of the pickup roller moved downward is changed depends on accuracy of the coil spring.